In the use of nuclear fuels based on oxide, particularly uranium oxide, one of the problems caused is due to the release of fission gases in the fuel element during the operation of the reactor, because these fission products must be kept in the fuel element, particularly in the actual fuel pellets, so as to limit the internal pressure of the sheaths or cans and the interaction of the fission products with the latter.
Therefore, at present, the burn-up of nuclear elements is limited to 50 GWj/t of U in order not to exceed the threshold beyond which the release of fission gases becomes significant.
However, operators of electronuclear reactors, particularly pressurized water reactors (PWR), wish to optimize control of the nuclear fuel by increasing burn-up of the uranium dioxide pellets contained in the rods in order to achieve minimum values of 60 to 70 GWj/tU.
Research carried out up to now for obtaining such an improvement has used procedures for increasing the size of the uranium dioxide grains, because it has been found that the gas quantity released by an irradiated large grain fuel is less than that released by an irradiated small grain fuel. Use has also been made of procedures for forming precipitates in the nuclear fuel in order to anchor the fission gases on such precipitates.
In order to obtain an increase in the size of the uranium dioxide grains, it is possible to add additives such as iO, NbO, CrO, AlO, VO and MgO to the uranium dioxide powder subject to fritting or sintering in order to activate its crystal growth, provided that the sintering takes place under a wet hydrogen atmosphere so that the added oxide quantity remains in solution in the uranium dioxide and is not reduced to a metallic element. The use of such additives for obtaining a large grain microstructure is, e.g., described by Killeen in Journal of Nuclear Materials, 88, 1980, pp 177-184, Sawbridge et al in Report CEGB RD/B/N 4866, July 1980 and Radford et al in Scientific Paper 81-7D2-PTFOR-P2, 1981. However, the use of certain additives of this type can lead to an increase in the diffusion coefficients of cations and fission gases in the uranium dioxide, which is unfavorable for the retention of the fission products and does not make it possible to take full advantage of the large grain microstructure.
Another procedure for improving the retention rate of nuclear fuels consists of dispersing in the uranium dioxide grains nanoprecipitates of a second phase for ensuring the anchoring of the fission products on such second phase. Nanoprecipitates of this type can consist of magnesium oxide inclusions, as described by Sawbridge et al. in Journal of Nuclear Materials, 95, 1980, pp. 119-128, and in FR-A-2 026 251.